Typical ground line sensors used to detect pressure changes in the earth, such as acoustic or seismic variations and signals, utilize several different types of transducer phenomena. Some utilize a sensitized or polarized teflon material producing an electret self-generating effect. Others use material giving a piezoelectric effect such as ceramic or quartz, and some a magnetic sensitivity is utilized. A third, and the type which this invention relates to is the impedance effect line. This operates in a similar manner to the electret line except no self-generating mechanism is present. A bias voltage is necessary so that as the capacity or impedance of the line changes (due to small flexures in position and over its length) a total change in capacity inductance or resistance occurs which in turn causes a current flow through a termination resistance. Past and present systems use a constant bias voltage across an offset coaxial line which results in the total line acting as a single capacitor. This causes a loss of any sensitivity in range along the line or where on the line the capacitance change occurred.
Present state-of-the-art ground line sensors that are designed to detect seismic and acoustic signals or disturbances through the earth, have bad limitations in range resolution. They also suffer in sensitivity and usefulness due to the masking effect of the total clutter and background noise picked up by the total length of the line. This limits the length a line can be made useful between detector terminations. This new invention has the unique capability to resolve range along the length of the line or cable to a degree approximately equal to the inverse of the cable's or structure's bandwidth. This also eliminates from the detection problem all clutter not in the same small range cell as the target, thereby allowing a very long line to be used with a single detector terminal, and much improved performance. This is accomplished by using a pulse of bias energy instead of a continuous bias on a line or structure that is designed to be bias dependent for its operation. The range correlated return can then be displayed or range gated for operational readout. This invention is not limited to just basic simple pulse waveforms but could use any time sensitive waveform such as linear frequency modulation, phase coding, pulse compression, spread spectrum, moving target indication (MTI), etc.
This invention will allow much more accurate determination of target range, while at the same time improving the detectability of weak target signals being masked by background clutter and noise.